Rotary drill bits are known in which a plurality of fixed cutting elements are used to cut through a rock formation, thereby forming or enlarging a bore. In some drill bits, cutting elements comprising polycrystalline diamond compacts (PDCs) and/or thermally stable polycrystalline (TSP) diamond are used. In such arrangements, a bit body is typically provided which is shaped to define a series of upstanding blades. A plurality of cutting elements is mounted to each blade, for example by being brazed into pockets formed thereon. Drill bits of this type are described in, for example, U.S. Pat. No. 4,554,986. Such polycrystalline diamond cutting elements provide high rates of penetration, but can be prone to damage and wear, particularly when drilling conglomerates or formations that include chert. An alternative to the use of polycrystalline cutting elements is to use a diamond impregnated metal matrix material in the formation of the bit body so as to form the body with regions serving, in use, as cutters to remove formation material. Drill bit of this general type are described in U.S. Pat. No. 6,843,333 and U.S. Pat. No. 6,510,906. Diamond impregnated cutting regions are typically tougher than PDC cutting elements, but have lower rates of penetration.
U.S. Pat. No. 6,296,069 describes a drill bit having polycrystalline diamond compact cutters mounted on the blades thereof, and having thermally stable synthetic diamonds and impregnated diamonds located elsewhere on the bit body. U.S. Pat. No. 6,009,962 discloses a drill bit comprises a bit body defining a series of blades with cutting elements mounted upon the blades, the leading surfaces of the blades being impregnated with abrasive particles.
WO2011/057303 describes an arrangement in which backup cutters are arranged behind and to follow associated primary cutters at a reduced cutting height.
It would be desirable to improve drill bit performance, for example by enhancing wear characteristics and rates of penetration. A drill bit that is more suitable for drilling challenging formations such as conglomerates or formations including chert is also desirable.
U.S. Pat. No. 7,798,256 describes the provision of an ultrahard material coating provided on the bit body, blades and gauge pads of a drill bit.